The present invention relates to a lightning protected fence controller system and method. More particularly, the present invention relates to a lightning protected electric fence controller system and method for protecting electric fence controller circuitry from induced voltage surges fed back along the electric fence line as a result of lightning strikes.
Electric fence controllers have long been used for controlling the charge on an electrified fence. As is well known; see for example U.S Pat. Nos. 3,655,955; 3,868,545; and 3,988,594, electric fence controllers are designed to impress an electric charge on a fence containing an electrically conducting wire, often made of an iron metal. The electric charge typically lasts only milliseconds and is repeatedly applied at predetermined spaced time intervals.
A common cause of damage to electric fence controllers are lightning strikes during thunderstorms which cause high voltage transients that damage the circuitry of the electric fence controller. Such high voltage transients are typically generated by lightning striking power lines, the electric fence wire, or adjacent to the electric fence wire, thereby inducing high transient voltages which eventually reach the circuitry of the electric fence controller. Thus, there is a need for protecting electric fence controllers against such lightning strikes. The previously referenced three patents all make reference to provisions in their design for protecting the electric fence controller from such lightning strikes.
With advances in electric fence controller design, fence construction, and insulation, very long multi-wire electric fences are now common, with total wire lengths of fifty or more miles not uncommon. As the length of electric fences and number of wires used increases and as the isolation of these wires from ground improves, the problem of lightning strikes also increases. The modern farmer or other user of electric fences needs a more reliable electric fence controller that will survive repeated high voltage surges induced by lightning strikes.
Some of the typical approaches currently used to reduce lightning related failure of electric fence controllers includes (1) fitting a simple spark gap across the electric fence controller output terminals, (2) installing a series choke/spark gap between the electric fence controller and the electric fence, (3) installing one or more spark gap diverters at points along the electric fence, (4) making the top wire of a multi wire fence a ground (shield) wire, and (5) protecting semiconductors with metal oxide varistors.
None of the above approaches have proved completely effective. One through four rely on the user installing a very low resistance ground rod and a high current, critically adjusted spark gap. Frequently, the "user installed" ground rod is ineffective and a high voltage induced surge will travel back down the electric fence line on all wires, live and ground, seeking the lowest resistance path to true ground.
In the case of a power line operated controller, the power line itself is a much lower resistance path to ground than most "user installed" local grounds, this is especially so when the ground temperature drops or is frozen and the soil resistance is very high. A high voltage surge can exceed 50,000 volts and on reaching the controller terminals, a dielectric breakdown (flashover) occurs within the electric fence controller, arcing across from the fence terminal components to the AC line input components. This flashover will destroy all or most semiconductors, burn out transformers, and in the extreme case, cause the controller to actually explode due to rapid expansion of the entrapped gases in the controller due to the heating effect of the high current arc discharge.
As regards the fifth approach, metal oxide varistors provide good protection to semiconductors, but very large and expensive metal oxide varistors are needed to withstand the extremely high energy levels induced in electric fences during a severe thunderstorm.
Lightning induced surges are also common on AC power lines and account for many of the lightning related failures of electric fence controllers.
The present invention provides an improved system and method of lightning protection for electric fence controllers due to lightning induced surges on both the fence wire and on the AC power line.